Treatment with Concanavalin A (Con A), a T cell mitogen, was found to induce suppresson of the antibody response to Type III pneumococcal polysaccharide (SSS-III) which was identical in both degree and kinetics to that seen after the induction of low-dose paralysis; this means that both forms of unresponsiveness - which are T cell dependent - share a common pathway for their expression and are mediated by suppressor T cells. Treatment with Con A also activates amplifier T cells, but only when the lectin was given 2 days after immunization with SSS-III; enhancement was simular in kinetics to that seen after treatment with antilymphocyte serum. Data was obtained to indicate that amplifier T cells act by driving B cells to proliferate further following antigenic stimulation and that suppressor T cells act on both amplifier T cells and B cells to limit the magnitude to the antibody response produced following immunization. BIBLIOGRAPHIC REFERENCE: Markham, R.B., Stashak, P.W., Prescott, B., Amsbaugh, D.F., and Baker, P.J.: Effect of Concanavalin A on lymphocyte interactions involved in the antibody response to Type III pneumococcal polysaccharide. 1. Comparison of the suppression induced by Con A and low-dose paralysis. J. Immunol. 118: 952-956, 1977.